The present invention relates to reducing the height of a raster output scanning (ROS) system and, more particularly, to using multiple, shorter focal length, wobble correction optical elements in the raster output scanning (ROS) system to reduce the ROS height.
Printing systems utilizing lasers to reproduce information are well known in the art. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoreceptor to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic light. This laser beam is modulated in conformance with an image information data stream by either an external acousto-optic modulator or by internal laser diode driver electronics. The modulated beam is transmitted through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive medium. The rotation of the polygon causes the spot to scan across the photoreceptor in a scan (i.e., line scan) direction. Meanwhile, the photoreceptor is advanced relatively more slowly than the rate of the scan in a slow cross-scan direction which is orthogonal to the scan direction. In this way, the beam scans the photoreceptor recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with the input image information serial data stream so that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photoreceptor to form a latent image, which is then transferred to an appropriate image receiving medium such as paper.
While raster output scanner based printing systems are well known, implementing such printing systems that fit into a small space or on a desk is difficult. One reason is the optical cross-sectional area of the raster output scanner. This optical area must remain obstruction free so that the charged photoreceptor can be properly illuminated which limits how small the printing systems can be. Raster output scanner designs which reduce the optical cross-sectional area are exceedingly useful.
A compact design for the scanning optics of these prior art type of ROS systems is desirable to make the machine itself as compact as possible and to enable extension of the same ROS design into many machine architectures.
One well known technique to reduce the size of a ROS system is to introduce folding mirrors to fold the optical path and allow the optical components to be positioned in a more compact area.
Prior art raster output scanner based printing systems often use mirrors to fold the laser beam onto the photoreceptor. Folding is beneficial since the optical path length can remain relatively large while the physical length of the path is reduced. Reflecting the laser beam with folding mirrors prior to sweeping the laser beam with the rotating polygon mirror is relatively straightforward. Using folding mirrors after the laser beam is sweeping after reflection from the rotating polygon mirror becomes more difficult since the resulting scan line must have a direction substantially perpendicular to the motion of the photoreceptor surface.
It would be desirable to improve the efficiency, shorten the optical path lengths, and use as few optical elements as possible to decrease hardware, assembly and alignment costs in a ROS system.
A typical prior art raster output scanning system 10 of FIG. 1 consists of a pre-polygon mirror optical section 12, a rotating polygon mirror scanning element 14 comprising a plurality of reflective facets 16, and a post-polygon mirror optical section 18 to correct for wobble of the rotating polygon mirror and to focus the beam along a scan line on the photoreceptor 20.
A light source, 22, such as a laser diode, emits a modulated coherent light beam 24 of a single wavelength. The light beam 24 is modulated in conformance with the image information data stream contained in the video signal sent from image output control circuit 26 to the light source 22.
The modulated light beam 24 is collimated by a collimating lens 28 in both the scan and cross-scan planes.
The collimated light beam 24 is focused by a cross-scan cylindrical lens 30. The lens 30 is cylindrical in the cross-scan plane and piano in the scan plane. Thus, the lens converges the cross-scan portion of the beam 24 focusing it on a reflective facet 16 of the rotating polygon mirror 14 but allows the scan portion of the beam 24 to remain collimated when the beam 24 strikes the reflective facet 14.
The collimating lens 28 and the cross-scan cylinder lens 30 are usually the only optical elements in the pre-polygon mirror optical section 12.
The polygon mirror 14 is rotated around its axis of rotation by a conventional motor (not shown), known to those of ordinary skill in the art.
The beam 24 reflected from the facet 16 is still collimated in the scan plane and is now diverging in the cross-scan plane. After reflection from the reflective facet 16, the beam then passes through post-polygon optical section 18, consisting of the f-theta scan lenses 32 and the anamorphic wobble correction lens 40.
The f-theta scan lens 32 consists of a negative plano-spherical lens 34, a positive piano-spherical lens 36, and the cross-scan cylinder lens 38. This configuration of f-theta scan lenses has sufficient negative distortion to produce a linear scan beam. The light beam will be deflected at a constant angular velocity from the rotating mirror which the f-theta scan lens optically modifies to scan the surface at a constant velocity.
The f-theta scan lens 32 will focus the light beam 24 in the scan plane onto the scan line 42 on the photoreceptor 20. The f-theta scan lens 32 only has optical power in the scan plane so the f-theta scan lens 32 will not effect the divergence of the light beam 24 in the cross-scan plane.
After passing through the f-theta scan lens 32, the light beam 24 then passes through a wobble correction anamorphic lens element 40. The wobble correction optical element can be a lens or a mirror and is sometimes referred to as the "motion compensating optics". The purpose of optical element 40 is to correct wobble along the scan line generated by inaccuracies in the polygon mirror/motor assembly.
The wobble correction lens 40 focuses the light beam in the cross-scan plane onto the scan line 42 on the photoreceptor 20. The wobble correction lens 40 only has optical power in the cross-scan plane so the wobble correction lens 40 will not effect the convergence of the light beam 24 in the scan plane from the f-theta scan lens 32.
The optical path length, and consequently the overall size of a rotating polygon ROS, is largely determined by the focal lengths of the lenses used to focus the beam onto the polygon and thence onto the scan line.
As shown in FIG. 2, in the side view in the cross-scan plane, the light beam 24 is reflected from the facet 16 of the polygon mirror 14 as a point 44. The light beam 24 will then diverge at a divergence angle 46 along the optical path 48 through the f-theta scan lens 32. The f-theta scan lens 32 only has optical power in the scan plane so the f-theta scan lens 32 will not effect the divergence of the light beam 24 in the cross-scan plane. The light beam 24 will diverge until the wobble correction lens 40 which then focuses the light beam 24 at a convergence angle 50 in the cross-scan plane to a point 52 on the scan line 42 on the photoreceptor 20. The point 52 at the photoreceptor 20 is at the focal length 54 from the wobble correction optical element 40, i.e., the distance from the optical element 40 to the point 52. The light beam 24 is at its maximum height 56 in the post-polygon optics 18 at its maximum divergence along the optical path 48 at the wobble correction optical element 40.
The overall height requirement of a ROS optical system 10 is typically dependent upon the resolution and the focal length of the wobble correction optical element. In other words, working backwards from the spot 48 on the photoreceptor 20, the beam 24 converges according to the spot size until the beam reaches the wobble correction optical element 40. This convergence angle and the distance the beam travels until meeting the wobble correction optical element determines the height of the beam at the wobble correction optical element. The beam diverges from the polygon mirror to the wobble correction lens then it converges from the wobble correction lens to the photoreceptor.
It is an object of the present invention to reduce the height of a raster output scanning (ROS) optical system.